The major goal of this proposal is to establish the qualitative and quantitative relationships between estrogen receptors (type I sites) and secondary estrogen binding sites (type II sites) in mammary tumor tissue. Methods for measurement and evaluation of binding site in cytosol and nuclear compartments will be examined in hormone dependent and independent mammary tumor lines. The nuclear localization and physical-chemical characteristics of type II sites will be determined. The influence of progesterone on type I and II estrogen binding sites will also be examined and the physiological interactions between these two steroids will be examined.